The invention relates to a method as well as a device for reproducing originals, such as photographic negatives, onto light-sensitive material such as photographic printing paper.
The exposure range of negative film and of CCDs of digital cameras is significantly greater than that of photographic paper. For this reason, it is very problematic to reproduce subjects rich in contrast and with great differences in brightness as a good print on photographic paper. While such subjects with very bright and very dark areas are captured by the negative film or the digital camera, on the paper print they are either underexposed in the bright areas or overexposed in the dark areas. It is then very difficult or impossible to recognize individual details in the underexposed or overexposed areas of the paper picture.
To improve the paper image quality of such contrast-rich pictures during photographic printing it is known to use masks to reduce the contrast when making prints.
For example, in the published German Patent application No. DE-OS 40 40 498 it is recommended to insert a mask that reproduces a very unsharp inverse projection of the original into the exposure light path of the negative during the printing process. This reduces the large area contrast in the print or copy while the contrast in the details remains unchanged.
The German Patent No. DE-PS 196 34 148 describes how the image original, the exposure apparatus and the mask can be attuned to one another and to the various printers. To compute the mask, an unsharp luminance image is created from the values y.sub.l,m of the luminance image stored in a two-dimensional matrix. ##EQU1##
Here, n is the number of points of the luminance image to be used for averaging. This unsharp image is now inverted and normalized, such that the final image points y'".sub.l,m for the mask are computed according to the equation: ##EQU2## taking into account the gradation. In this equation, y".sub.d stands for the value of the darkest image point of the unsharp luminance image and .lambda. for the gradation factor.
In a digital copying process, the mask is computed together with the luminance image such that the corrected image points of the luminance image are the result of ##EQU3##
The mask changes the printed image such that, for example, a dark area in a light surrounding is brightened such that the patterns in the dark area become more visible. In this manner, the image information, for example in the area of the direct shadows falling onto bright faces, is brightened such that the areas of the faces located in the shade can be clearly recognized. The gradation factor .lambda. is responsible for the degree of brightening, or more general, for the strength of the mask--that is, the degree of reduction of the large area contrast--of the contrast between two large areas of different brightnesses.
The German Patent No. DE-PS 196 34 148 suggests to determine whether masking should be used based on the density volume of the original. Once a decision for masking has been made, each image is overlaid with a mask with a pre-determined, computed gradation factor. The same gradation factor .lambda.=0.5 that has been proven as advantageous is used for all images.
It has been found that the method described in the aforementioned German Patent No. DE-PS 198 34 148 leads to excellent picture results with some originals, however, with other originals the result is not satisfactory. For example, it has been noticed that the image impression is significantly worsened in comparison to unmasked prints, if black spots, that is, spots in the negative that are not exposed and that contain no image information, are brightened during printing exposure. The color tipping of the film can cause the completely black areas of the original to appear in the print as disturbing structure-less areas in gray, green or brown. For example, it may be disadvantageous to the image result if pictures are masked where the effect is created by the contrast between bright and dark--for example a frame of a picture showing a black cat in front of white wall. In the print, the cat would appear gray if the image were masked according to the method described.